A simple implementation of a transmitter concept for transceivers in mobile radio systems is offered by transmitters which have a modulator that operates on the known principle of two-point modulation. A PLL (phase locked loop) circuit is in this case used as a frequency synthesizer, and is used for phase modulation or frequency modulation of a radio-frequency signal.
The modulation signals are normally injected at two points in the PLL circuit. Firstly, a programmable frequency divider in the PLL circuit is driven by a digital modulation signal. The programmable frequency divider is arranged in the feedback path of the PLL circuit and represents a point in the PLL circuit which has a low-pass transmission response for the injection of modulation. The digital modulation signal may in this case have a wider bandwidth than the low-pass filter that is formed by the PLL circuit. Secondly, an analog modulation signal is injected into a summation point which is located in the forward path of the PLL circuit and is preferably connected upstream of the voltage controlled oscillator. The analog modulation which is fed in at the summation point has a high-pass filtering effect, by means of the closed control loop, on the output of the PLL circuit, so that the corresponding modulation signal is in turn corrupted by the transmission response. The digital modulation signal and the analog modulation signal are superimposed at the output of the PLL circuit, and this results in the PLL circuit having a transmission response which is independent of frequency. The simultaneous injection of a digital modulation signal and an analog modulation signal into a PLL circuit is referred to as two-point modulation.
A two-point modulator such as this and a method for phase modulation or frequency modulation using a PLL circuit is described in German Laid-Open Specification DE 199 29 167 A1. A digital modulation signal is fed into the control connection of a frequency divider in the feedback path of the PLL circuit, thus resulting in the determination of the number by whose reciprocal the instantaneous frequency of the input signal to the frequency divider is multiplied. Furthermore, the digital modulation signal is converted by means of a digital/analog converter to an analog modulation signal, which is injected into the PLL circuit at a summation point which represents a high-pass filter point.
In the described type of transmitter concept, the control loop remains closed. In order to achieve a low noise level in the PLL circuit, the bandwidth of the PLL circuit is designed to be considerably narrower than would be necessary for transmission of the modulated data. In order to compensate for the narrow bandwidth, the analog modulation signal is injected into the PLL circuit, as well as the digital modulation signal.
The essential feature for the method of operation of two-point modulation is that the digital modulation signal and the analog modulation signal have amplitudes which match well, in addition to being in phase. However, the production tolerances in the components which are required for analog modulation result in fluctuations in the modulation gradient and in the amplitude level of the analog modulation signal. For this reason, after production of the PLL circuit, amplitude trimming must be carried out between the analog modulation signal and the digital modulation signal.
If temperature influences are also intended to be taken into account, such trimming must be carried out before each transmission process.
One known method for trimming a PLL circuit with two-point modulation is to apply the two-point modulation to the circuit in the steady state, and to receive and to demodulate the transmitted signal using an external measurement receiver. The digital and analog modulation signals are trimmed as a function of the demodulation result that it is obtained. Owing to the non-linear frequency response of the oscillation-generating element of the PLL circuit—a voltage controlled oscillator (VCO—as a function of the control voltage, this trimming process must, however, be carried out for each channel. This results in a correspondingly long measurement time when there are a relatively large number of channels. In addition, the trimming information must be stored in a memory. A further disadvantage of this method is that the influence of temperature changes is ignored in this method.
A further known method for trimming a PLL circuit with two-point modulation is essentially based on the method described above. However, in this case, the reception and the demodulation of the signal that is produced by the PLL circuit are carried out by the receiving section in the transceiver. However, this involves considerably more circuit complexity, since a second PLL circuit is required in the receiver in order to carry out this method.